Canadian Medical Association Journal 1996; 154: 1884-1888
© 1996 Canadian Space Agency
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En bref
Robert Thirsk, médecin de famille et un des premiers membres du programme des astronautes de l'Agence spatiale canadienne, fera partie de l'équipage de sept membres de la navette spatiale Columbia qui décollera du Kennedy Space Centre, en Floride, le 20 juin. Dans ce rapport spécial, le diplômé membre de la promotion 1982 de McGill décrit certaines des expériences en physiologie et en science des matériaux que l'équipage réalisera. Spécialiste de charge utile et médecin de l'équipage, le Dr Thirsk pense que les résultats pourraient avoir un impact important sur les futures missions spatiales, la médecine et l'industrie de la biotechnologie.
This dream began in 1983 when I was named one of the original members of the Canadian Astronaut Program. At the time I was a second-year resident in family medicine at the Queen Elizabeth Hospital in Montreal. A year later, as backup astronaut for shuttle mission 41-G, I got a taste of what it would be like to participate in a space mission. However, training is not the same as actual flying. I am delighted and feel very fortunate to be chosen for a mission that seems uniquely designed for my interests and abilities.
My six LMS crewmates, our two backups and I represent five international space agencies -- NASA and the French, European, Italian and Canadian agencies. We have been training for the last 15 months to prepare for 16 days in space aboard the space shuttle Columbia. On behalf of scientists from the US, Europe and Canada, we will perform 43 investigations devoted to the study of life and materials sciences. The life-sciences experiments will probe changes in plants, animals and humans under spaceflight conditions. The materials-science investigations will examine protein crystallization, fluid physics and high-temperature solidification of multiphase materials in a microgravity environment. (Microgravity describes experimental conditions in the shuttle, which are not perfectly weightless. As the shuttle orbits Earth, it is subjected to small decelerations from atmospheric drag. Other factors, such as the location of experiments relative to the shuttle's centre of gravity and vibrations from spacecraft machinery induce small accelerations on the order of one millionth the force of Earth's gravity, or one microgravity.)
Our initial training took place in the investigators' laboratories and at NASA centres, where we were instructed in the theory, hardware and operations involved in the experiments. Training for the flight took place at the Marshall (Alabama) and Johnson (Texas) space centres, where in-flight operations involving ground controllers were realistically simulated in high-fidelity spacecraft trainers. Everyone involved in the mission -- scientists, ground controllers, astronauts -- participated in several multiday simulations to rehearse planned operations, communication procedures and problem-solving techniques. Thorough training is essential, because in human space flight we have only one in-orbit chance to perform the experiments properly.
Due to my clinical background I have also trained as one of the designated crew medical officers, and will diagnose and treat any in-flight medical problems in consultation with the flight surgeon on the ground.
Fortunately, significant illness and injury have been uncommon on shuttle flights. Rigorous medical screening of candidates ensures that astronauts are in good health. Furthermore, a 7-day quarantine period prior to launch minimizes crew exposure to infectious diseases, and the relatively short duration of shuttle missions -- it ranges from 7 to 16 days -- reduces the probability of in-flight illness.
Minor illnesses such as space motion sickness, nasal congestion, headache and backache are common. More serious problems, such as renal colic, chronic prostatitis, pneumonitis, cardiac arrhythmias, decompression sickness and corneal abrasions have been rare occurrences on other spacecraft and have had a serious impact on mission objectives.
A well-stocked medical kit will allow me to deliver ambulatory care, first aid and basic life support. It contains oral, topical and injectable drugs, intravenous fluids, bandages and the equipment and supplies needed to treat routine medical and dental problems. A contaminant clean-up kit contains equipment to protect crew members from toxic substances and a pair of eye goggles that can be used to flush contaminants from the eyes.
I am eagerly anticipating the bone-rattling launch, the tranquillity of weightlessness and gazing in awe at vast regions of our planet. I am equally excited by the broad range of human physiological investigations we will conduct, which will occupy most of our work time. Spaceflight results in significant physiological and biochemical changes in every organ system. My crewmates and I will be both onboard researchers and subjects for several experiments investigating these changes. Most LMS scientific investigations will be conducted in the Spacelab laboratory. This cylindrical pressurized module in the cargo bay provides a shirt-sleeve working environment that includes work stations, experiment facilities, freezers, storage compart- ments and other support equipment. I have an interest in all these experiments and feel I am part of the pioneering effort to understand mechanisms behind the adaptive changes. I will outline a few of these LMS experiments.
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Torso-rotation experiment
The torso-rotation experiment (TRE) is the only one from Canada; Dr. Douglas Watt of McGill University's Aerospace Medical Research Unit is the principal investigator. The idea was hatched several years ago when one of Watt's colleagues suffered an injury that required him to wear a rigid neck brace. He began to feel nauseated after several minutes moving about his laboratory with his head braced to his torso.
Voluntarily fixing the head to the torso, as with a neck brace, has been termed "torso rotation," since the upper body must move to turn the head. On the ground this gradually leads to motion sickness in most subjects. Torso rotation is an example of a deliberate "egocentric" motor strategy, in which the subject concentrates on a body frame of reference rather than an external world reference. A motor strategy similar to torso rotation is often inadvertently adopted by astronauts during the early days of a space mission. While this may be a means of reducing head movement to combat motion sickness, it might actually exacerbate the symptoms.
Watt's aim is to monitor our eye, head and torso movements for evidence of egocentric motor strategies as we perform typical in-orbit activities. Eye movements will be measured with electro-oculogram (EOG) electrodes, and head and torso movements with angular velocity transducer units securely mounted on the top of the head and upper back. Each of us will perform the TRE three times during the mission -- a few hours after launch, halfway through the mission and near the end of the flight.
The data will be analysed for evidence of unusual gaze control during coordinated eye-head movements. As well, head rotation will be compared with torso rotation to assess our overall motor strategy and how it might change during the mission. If motor activities similar to torso rotation contribute to motion sickness in space, it would be relatively easy to train future astronauts to avoid such movements or to pre-adapt them by mimicking the provocative movements under controlled conditions.
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Skeletal-muscle experiments
Previous missions have shown that microgravity produces significant skeletal muscle wasting, particularly in the postural muscles. The extent and responsible mechanisms are only partially understood, and this is a medical concern during extended stays aboard space stations or for future missions to the inner planets. Crews returning from long missions likely will show decreased muscle strength and endurance, which will pose health and safety concerns.
This will be the first space mission to investigate comprehensively the changes in human skeletal muscle structure and function induced by the weightless environment. The LMS work is actually an aggressive and coordinated effort involving six research teams from the US and Europe. They will investigate both flexor and extensor muscle groups acting at the ankle, knee, wrist and elbow. They will provide a detailed analysis of the relative importance of weightlessness-induced changes in motor perception, recruitment and hormonal and cellular factors in mediating the reduced performance of these limb muscle groups.
A sophisticated torque-velocity dynamometer facility has been developed to support the in-flight investigations. It measures the torque and the angular velocity produced during predefined types of contractions of the flexor and extensor muscles of the elbow and the ankle. These two measurements enable investigators to quantitate several muscle-performance and function properties.
We will test the hypothesis that the primary factors reducing limb-muscle function can be attributed to alterations at the cellular level, specifically a selective loss in the contractile proteins. We expect that space flight will result in increased muscle fatigability due to preferential atrophy of slow-twitch muscle fibres, coupled with a percentage increase in the more fatigable fast-twitch fibres and an inhibition in the ability of muscle cells to metabolize fats. Furthermore, a greater atrophy of predominantly slow-twitch muscles such as the soleus compared with fast muscles such as the gastrocnemius may be detected by changes in muscle volume when postflight magnetic resonance images of our muscles are compared with preflight images.
Data from earlier spaceflights suggest that the decrease in limb-muscle strength is out of proportion to the corresponding degree of muscle atrophy, so other mechanisms also may play a role. We will examine the possibility that at least a portion of the decline in muscular strength is caused by alterations in motoneuron function or motor-fibre recruitment.
Muscle biopsy samples will be obtained from the gastrocnemius and soleus muscle of each participating crew member. The mechanical, morphologic and metabolic properties of the muscle fibres that are biopsied postflight will be compared with those obtained preflight.
Because of the electromyograph (EMG) and percutaneous-stimulation electrodes that we will be wearing and the number of blood and tissue samples that will be taken to support the muscle physiology investigation, we lightheartedly refer to ourselves as the "rat crew" (as in laboratory rats)! However, the LMS investigators hope to gain a deeper understanding of the effect of weightlessness on muscle function and to provide useful recommendations to improve both rehabilitation programs in clinical situations where musculoskeletal unloading is a component and countermeasures for preventing spaceflight deconditioning.
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Pulmonary function
Our lungs are also sensitive to gravity. The mass of lung parenchyma, airways, chest wall and diaphragm causes them to deform under their own weight. As well, hydrostatic gradients within the body influence intrathoracic blood volume, the distribution of pulmonary blood flow and the return of blood to the right side of the heart. During the LMS mission we will perform a series of noninvasive pulmonary-function tests to gain understanding of the intrinsic behaviour of the lung in a unique laboratory setting unaffected by the force of gravity.
We will utilize a computerized pulmonary-function-testing system built around three inspiration and exhalation bags, a variety of test gases, flow and volume measurement systems and a respiratory mass spectrometer. Specific tests designed at the University of California at San Diego will evaluate resting gas exchange, distribution of ventilation and perfusion, diffusing capacity, lung volumes and fluid content, cardiac output, mechanisms of gas mixing in the lungs, ventilatory control mechanisms and respiratory mechanics.
Little is known about the cardiopulmonary system's response to heavy exercise in microgravity. Such knowledge is important when astronauts work hard during space walks and while performing other duties during extended tours in space. For this reason we will perform the pulmonary-function tests at rest and shortly after completing a session of heavy exercise on a cycle ergometer.
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Sleep and circadian studies
Humans have a biological clock that ensures we are physiologically and psychologically prepared for active wakefulness or restful sleep at appropriate times. Our preparation is accomplished by circadian rhythms that are generated, at least in part, by a self-sustaining master oscillator located near the suprachiasmatic nuclei. However, abrupt changes in environmental factors or daily routine can result in misalignment of our biological clock relative to our activities.
The environment and routine associated with life in space are quite different from those on Earth. Spaceflight is characterized by weightlessness, 16 sunsets and sunrises per day, social isolation, cramped living conditions and the absence of the Earth's 24-hour temporal structure. Over the duration of a mission astronauts might experience a progressive tendency towards circadian dysfunction, with resulting decrements in sleep and performance.
The aim of the sleep and circadian studies experiment from the University of Pittsburgh is to evaluate our sleep, circadian rhythms and task performance. For several nights the quality and quantity of our sleep will be evaluated by electroencephalograms, EOGs and EMGs, and computerized sleep diaries. Circadian rhythms will be measured by core body temperature and urinary variables (volume, sodium, potassium, melatonin, cortisol), and performance by cognitive-function tests and subjective reports of mood, vigilance, stress and workload. As human spaceflights become longer, the results from this experiment may have operational significance. Modification of work schedules and the provision of time cues may be recommended to maintain astronaut performance levels and feelings of well-being.
We will participate in several other physiological studies of neurovestibular adjustment, regulatory physiology, bone demineralization and cognitive function. On behalf of biologists, we will collect data on the adaptation of other living systems (plants, fish embryos and rats) to the microgravity environment. Together these experiments complement each other to provide a comprehensive view of the effects of space travel on living systems.
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Materials science
The materials-science experiments are less crew intensive because mostly they will be controlled automatically or commanded from the ground during the night while we sleep. During the day, we will change samples and film for these investigations.
Of interest to the biotechnology industry is the Advanced Protein Crystallization Facility (APCF). With recent refinements in the technique of x-ray crystallography, biochemists and molecular biologists now have a powerful tool for probing the structure of complicated macromolecules in three dimensions at the atomic level. The limiting factor in x-ray crystallography is no longer the technique itself but the availability of macromolecular crystals. Most of the thousands of known proteins and nucleic acids do not crystallize readily. Failure to obtain these crystals for visualization hinders basic understanding of macromolecular structure as well as the rational, systematic design of drugs intended to interact with target proteins and nucleic acids.
Protein crystallization on Earth is disturbed by gravity effects such as convection and sedimentation, resulting in the production of defects and reduced crystal size and quality. Small, inhomogeneous crystals are poor candidates for x-ray diffraction analysis. The APCF experiment will attempt to determine whether macromolecular crystals nucleate and grow larger and more uniformly in microgravity. Ninety-six separate experiments under controlled temperature conditions are accommodated in the facility. A video microscopy system will allow the facility's multiple users to observe the crystallization process in 24 of the reactor cells. The long-term goal is not to establish a protein crystallization facility in space, but to better understand crystal growth on Earth. Improvements in the crystallization process in conventional terrestrial labs may then accelerate advances in biotechnology, medicine and agriculture.
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Postscript
Since 1984 I have put my clinical career partially on hold. My work as an astronaut is a full-time commitment that focuses primarily on operational training, medical research and flight payload design. My employer, the Canadian Space Agency, recognizes that my clinical skills benefit the goals of the Canadian space program, and has facilitated my continued involvement in part-time medical practice and continuing medical education.
I miss contact with my patients and the challenges of diagnosis and treatment, but I am doing what I enjoy. My return to clinical medicine is postponed while I pursue present and future opportunities that are simply out of this world.